The diatom biostratigraphy of Holes 1095A, 1095B, and 1095C is complex and contains an incomplete record of Pleistocene through upper Miocene datums. The Pliocene to Pleistocene is represented by glacial and interglacial hemipelagic sediments, whereas the Miocene consists mainly of turbidites. Many core intervals are barren or have low diatom abundance and richness. In samples that contain diatoms, reworking of older species is often noted. In spite of the obvious reworking, there is a strong biostratigraphic signal in the lower Pliocene and upper Miocene.
All datums and their associated hole/core depths at Site 1095 are listed in Table T2. The identification of diatom species observed at this site follows the taxonomic concepts of McCollum (1975), Schrader (1976), Fenner et al. (1976), Gombos (1977), Akiba (1982), Barron (1985), Yanagisawa et al. (1989), Gersonde (1989, 1990), Yanagisawa and Akiba (1990), Gersonde and Burckle (1990), Baldauf and Barron (1991), Fenner (1991), Harwood and Maruyama (1992), and Gersonde and Bárcena (1998). More details on species taxonomy can be found in Iwai and Winter (Chap 35, this volume). Datum intervals are defined by the sample that contained either the first or last occurrence of that particular species and the sample immediately above (for a LO datum) or below (for a FO datum). The depths of these samples is given in meters below seafloor (mbsf). If the adjacent sample in the upcore or downcore direction (depending on type of datum) is barren, then the interval will only list one sample. Less confidence is placed on these datums, compared to those with a well-defined appearance or disappearance from the assemblage. Species abundance data can be found in Table T3.
The samples in Cores 178-1095B-29X through 45X contained a typical upper Miocene assemblage, with Denticulopsis species dominating. The species of Denticulopsis that are used as zonal boundary markers in older sediments, D. dimorpha and D. praedimorpha, were absent from the assemblage. One sieved sample, from Core 178-1095B-37X, contained Asteromphalus kennettii, which has been noted to have a FO age of 10.29 Ma in other studies. This datum also helps constrain the age of the lower part of Hole 1095B to the upper Miocene.
The neritic diatom Paralia sulcata is observed in samples from Cores 178-1095B-21X through 27X and is especially common in Cores 23X and 24X (290 to 310 mbsf) (see the "Biostratigraphy" and "Sedimentation Rate" sections in the "Site 1096" chapter of the Leg 178 Initial Reports volume [Barker, Camerlenghi, Acton, et al., 1999]). Paralia sulcata is a benthic neritic species that lives on coarse sediment in a relatively low-salinity shallow-water environment and can easily be transported to deep water. The transport from a shallow-water environment can occur through sediment transport or by an increased flow of a relatively low-salinity water from the shelf. The sporadic occurrence of benthic diatoms Navicula spp. and Pinnularia spp. and of the reworked middle Miocene diatom Denticulopsis dimorpha var. areolata in the same interval suggests shallow-water sediment transport. For the abundance of P. sulcata to have reached the observed proportion, it is possible that more of the shelf was in the photic zone and therefore shallower than today (M. Iwai, unpubl. data).
Compared with Site 1095, Site 1096 exhibits both increased sedimentation rates and better biogenic preservation. Calcareous nannofossils were unexpectedly recovered in material from this site within the upper ~170 mbsf. Below this depth, the biostratigraphy is primarily based on the diatom record. The change at 170 mbsf coincides with the lithologic change from Unit II to Unit III, which continues to the base of Hole 1096C. High sedimentation rates in this lower section expanded the biostratigraphic zones to approximately three times their thickness at Site 1095. The age of the bottom of Hole 1096C is believed to be early Pliocene.
Diatoms are generally rare to barren in sediments from Hole 1096A and the upper parts of Holes 1096B and 1096C. The exception to this is noted in brown-colored, possibly interglacial sediments occurring in the lower portion of Cores 178-1096A-2H and 178-1096B-2H and 3H in which diatoms are common and the assemblage is dominated by Fragilariopsis kerguelensis, Thalassionema spp., and Thalassiothrix spp. fragments. The occurrence of Hemidiscus karstenii also suggests that those core intervals are within 0.19 to 0.49 Ma (Gersonde and Bárcena, 1998). The datums observed at this site are given in Table T4, and species abundance data are given in Tables T5, T6, and T7.
At Site 1101 we recovered 217.7 m of Pleistocene and Pliocene hemipelagic silty clays. All four microfossil groups (diatoms, radiolarians, calcareous nannofossils, and foraminifers) were present. Calcareous microfossils were observed in the uppermost core as well as in a carbonate-rich unit from 50 to 134 mbsf. Siliceous microfossils were seen throughout the hole but are more abundant in the three lowest cores.
Diatom preservation and abundance varied throughout Hole 1101A. Intervals of increased well-preserved diatom material alternated with barren zones. Fragmentation of valves is very common in all of the upper 160 m of Hole 1101A. Diatoms in samples below Core 178-1101A-19X were more abundant than in the upper part of the hole. The lowest three cores (Cores 178-1101A-22X, 23X, and 24X) contained an open marine, diverse assemblage of diatoms. All zones from the Pleistocene to the upper Pliocene were identified at Site 1101. Table T8 lists the diatom datums noted at this site; intervals of these datums have both their sample position within the cores and their depth (in meters below seafloor) given. The species abundance data for Site 1101 can be found in Table T9.
Further species datums discussion is included below in the "Discussion and remarks" section for each zone.